My feeling is that because I can buy 2 TB USB "spinners" for about $100 a pop, it makes perfect sense to take a couple of them on any journey; put a back-up on each one; and keep the drives in separate places (with one of them on my person).
These little HDDs seem to be perfectly stable and it is unlikely that both will fail simultaneously, if they fail at all, and they haven't done so — so far!
I download to the SSD in my MPB each night and then leave the computer running while it copies the files to the external drives so it doesn't matter if that takes a little longer than it did to do the original download through my card-reader.